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    <title>Title</title>
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    <!--响应流-->
    <img src="/verifycode/img">
    <!--响应base64编码字符串-->
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   输入验证码-->后台获取到对应输入验证码---->和session里面进行对比
  只要我们能从session中获取到验证码
  <a href="/verifycode/img/xxx">校验</a>
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